1. Field
This disclosure relates generally to the field of power generation using waste heat. More particularly, the disclosure relates to a system and method for recovering waste heat from a plurality of heat sources having different temperatures for generation of electricity.
2. Background
Significant amounts of waste heat are generated by a wide variety of industrial and commercial processes and operations. Example of sources of waste heat include heat from space heating assemblies, steam boilers, engines, and cooling systems. In general, waste heat is discharged into the environment, or occasionally used as a low grade heat source for space heating and the like.
In a typical system based on the waste heat of an internal combustion engine or other fuel-based heat source, the original source of heat (the engine) discharges heat in the form of exhaust and discharges an additional amount of heat in the form of mechanical cooling (engine cooling).
One method to generate electricity from the waste heat of a combustion engine is to apply a bottoming Rankine cycle. A Rankine cycle engine typically includes a water-based system including a turbo generator, an evaporator/boiler and a condenser; however, water based steam Rankine cycles are unattractive for low temperature waste heat systems due to high cost and low efficiency. The performance of an organic Rankine cycle (ORC) is limited by constraints of the working fluid circulated within the ORC. Any pure chemical used as a working fluid may be optimal for a specific limited range of cycle temperatures and pressures. It is therefore difficult to maximize the power output of a single fluid ORC for a system with waste heat sources of different temperature levels. The working fluid is a thermodynamic medium which functions as a working fluid in the system.
Externally Fired Gas Turbine (EGFT)/combined cycle systems have been described in the literature for a number of years. Such systems include a compressor for compressing ambient air, an indirect contact heat exchanger in which combustible products, e.g., gas and/or fuel vapors, hereinafter referred to as “combustible gases”, are burned to heat the compressed air, and a turbine in which the heated compressed air is expanded driving a generator that produces electricity. Heat contained in the turbine exhaust is used to vaporize water that is converted into steam in a separate water-based, closed Rankine cycle power plant, the steam being expanded in a steam turbine in the power plant for driving a generator that produces additional electricity.
Organic Rankine Cycle power plants have been proposed for operation together with such systems, see for example U.S. Pat. Nos. 5,687,570, 5,799,490, 6,167,706 and 6,497,090, the disclosures of which are hereby incorporated by reference. In these patents, while an Organic Rankine Cycle power plant is described which produces power as a bottoming cycle to a steam turbine power cycle operated by a gas turbine power unit, an Organic Rankine cycle is also described which produces power from an intercooler of a compressor of a gas turbine power unit, see for example FIGS. 5A, 6, 7, 8, 9 and 9A of the above patents. Furthermore, in these patents, while Externally Fired Gas Turbine (EFGT) combined cycle systems are described, combined cycle power plant systems are described with include gas turbines that can be directly fired as well, see for example FIGS. 8, 9, 9A.
EFGT systems have been proposed for use with low calorific, unclean gaseous fuels as well as with hot gaseous fuels. Solid fuels are more difficult to incorporate into EFGT systems because of the problems associated with ash and noxious gases produced during the combustion process.